This invention relates to couplings using standard A.P.I. tubings or casings, and more particularly to systems and methods for upgrading the applicable uses of standard tubings and casings under a variety of field conditions and for a variety of applications.
The oil and gas industry standardized many years ago on certain grades and types of tubing and casing, as a result of studies and the adoption of standards by the American Petroleum Institute (A.P.I.). As a result of their determinations, weights, grades and thread characteristics for tubing or casing (pins) and the intercoupling collars (boxes) were established. This afforded the degree of predictability needed as to when and what tubular goods could be used in a given downhole pressure and tension application. While conditions in the oil and gas fields are too variable and stringent to provide perfect assurance of feasibility in every situation, the standards were found to provide adequate reference points and reliable operation under most conditions under the circumstances then applicable. These circumstances involved shallower wells and less demanding environments than are presently encountered as the increasing search for oil and gas resources has carried drilling to far greater depths than formerly attempted. Nonetheless, A.P.I. EUE 8 inch round threaded and coupled tubing has been used by the industry for many years, and continues to be used widely. It is not only an A.P.I. standard but is available worldwide and this tubing and casing provides tough, rugged operation that will survive transporting, that is, rig floor handling and make-up, much better than its counterparts. In an upset (thickened end) version, the A.P.I. standard provides excellent tension and bending performance that can readily handle most load cycles with a long useful life. Furthermore, the tubular goods can be economically cut or recut, and can be repaired with readily available equipment in most applicable locales in the world. However, when it is desired to achieve a higher level of performance, the tendency is to use a "premium grade" of tubing, typically using special configurations of threads and seals, at substantially greater expense. Very often, such premium grade tubular goods must be processed specially with such precision that they can only be prepared in special factory installations, and must be transported back and forth in order to be kept in use. Consequently, the tendency has been to utilize this type of product only when its use is otherwise unavoidable.
The cost of tubing or casing alone, while very significant to the economics of oil and gas well production, is but a part of the overall expense involved. The grade and type of tubing needed for a well cannot be precisely ascertained until after the drilling has been carried out and the well logged. Only then can the operator be sufficiently assured of the down hole conditions to select a grade and type of tubular goods for use. An inventory of A.P.I. standard goods will be on hand, but it is uneconomic to maintain different premium pipes and a range of coated goods on site. Thus it has been necessary to transport goods (called "Movements") repeatedly for processing, including bucking off, bucking on and machining. The costs of these movements are a major factor in well production expenses. Adding an internal coating or a lining to A.P.I. pipe can often be done in the field because of the availability of equipment for this purpose.
A.P.I. tubing has certain limitations, resulting in part from the fact that it has leak paths designed into the thread configuration, which must be blocked by the internal thread lubricant, which contains solids for that purpose. The effectiveness of the damming action is variable and uncertain. In addition, the tolerances on the A.P.I. pins and boxes are relatively wide, so that a problem constantly exists in assuring proper make-up of the couplings. Under these circumstances, therefore, a separate industry using high technology approaches has been devised for instrumenting and monitoring pipe make-up, using torque measurements, seal checks, and other expedients. Nonetheless, the proper make-up remains dependent on the dimensions of the mating pin and box. With random assembly in the field, problems continue to exist.
Given these considerations, applicant has previously devised a coupling system for A.P.I. standard pipe in which the pins remain unchanged, so that the available tubing and casing inventory can be used, but a coacting collar having a central positioning ring, either integral with or insertable into the collar, together with inner and outer seals, is utilized. The seals are disposed adjacent to the reference shoulder defined by the inner ring, in a groove in the collar, and in a groove in the end recess portion of the collar. This arrangement provides a highly satisfactory form of coupling and enables the widespread usage of A.P.I. tubing on a reliable basis, with ready make-up characteristics.
The search for superior forms of coupling, however, is never ending. A variety of field problems of different natures are constantly being encountered. Analysis of the stresses acting on the coupling system using the inner and outer seals has revealed that certain limitations are presented because, even though the tubular goods are unmodified, the presence of the seal and the necessary grooves which were retaining them impose certain limitations. For example, the seal itself, when "energized" or stressed by the entry of the pin into the box, tends to deflect the pin laterally, especially when the system is pressurized. Additionally, the seal groove can allow, under these highly stressed conditions, bending at the collar. Furthermore, the seals, which must be of high temperature and high corrosion resistance, add substantially to the cost of the unit even though it is superior to the prior designs. The use of the integral collar or stop shoulder presents an additional problem, because the stresses interior to the coupling can thus be unequal, inasmuch as they are not transmitted from one pin to the opposed pin through the ring.
It has been ascertained, however, that the metal-to-metal contact in this configuration provides a degree of sealing under many conditions, in response to high pressure differentials. Investigating this further, it has also been ascertained that this metal-to-metal seal characteristic involves complex factors. Overstressing during make-up can lead to nonelastic deformation and galling. Without sufficient metal-to-metal preload stress, the completed string may totally relieve the metal-to-metal pressures, resulting in a leakage path. There are limitations on what can be done in this respect, inasmuch as the tong forces available are typically limited, and cannot exceed certain values in any event.
As another example of the conditions which must be met by a low cost coupling utilizing A.P.I. tubular goods in a pipe string, there are a variety of situations in which a given collar might be used. In some situations, it is satisfactory to leave the central area (the so-called "J- . area") between the pin ends open. In others, which might be called the "smooth bore" configuration, it is desirable to have a central ring only for dimensional control as well as providing a uniform flush interior diameter between the tubular goods and the intervening collar. In a more demanding application, it is desired to have a metal-to-metal seal that will retain its integrity under the highly stressed conditions that might be encountered. In a 12,500 foot well, for example, 8 inch round pipe of a typical grade will place 700,000 lbs. of tensile force on the uppermost collars, which must be borne by the threads at that position. Further, much A.P.I. standard pipe is internally protected in some way, because of the conditions to be encountered. Some are coated with a plastic liner by a spray infusion process, others are internally coated by an internal liner which adheres to the sides of the tubing and around the nose ends, and still others have a liner of high temperature cement. It is desirable that the mechanical, make-up and sealing characteristics be attainable using essentially the same collars, but employing only a minimum of variation, objectives which have not heretofore been achieved in any measurable degree with an economic coupling system. The A.P.I. standard defines a "last scratch" mark which is used as a dimensional guide for controlling markup under low stress, low pressure conditions. The vastly more demanding conditions which must now be met for many wells, together with the wide tolerances for A.P.I. goods, however, has required the use of torque measurements, post makeup testing, premium couplings, and other techniques. All of these add expense in the form of materials or testing costs. Dimensional makeup approaches have been recognized to be desireable, but heretofore the means have not been available for using dimensional control to use A.P.I. standard tubular goods for a variety of standard and upgraded conditions. By "dimensional control" is meant both physical references, such as makeup to a reference mark, the hand tight plane or a reference shoulder engagement, and makeup to a specified number of turns past such a reference.